Broadband receiving antenna



June 13, 1967 c. STROMSWOLD $325,814

BROADBAND RECEIVING ANTENNA Filed June 23, 1965 lNVENi'OA CHESTERSTROMSWOLD GL-am A TOR/V r United States Patent 3 325 814 BROADBAND RECE IWHNG ANTENNA Chester stromswold, Nashua, N.H., assignor to SandersAssociates, Inc, Nashua, N.H., a corporation of Dela- Ware Filed Juneas, 1965, Ser. No. 466,206 12 Claims. c1. 343]l18) This inventionrelates to a radio direction finder and to broadband radio frequencywave-intercepting devices particularly suited for use in a directionfinder.

A direction finder of the present type is used to determine the arrivaldirection of signals within a wide frequency range. In the prior art,however, as the frequency range is extended, the antennas and other waveintercepting devices of the direction finder increasingly react withintercepted energy. At certain frequencies, they reradiate theintercepted energy and thereby mask the original source. As a result,erroneous direction indications are produced.

Accordingly, it is an object of the present invention to provide animproved direction finder suited for wideband operation.

Another object is to provide a wideband direction finder in which thewave-intercepting devices operate with relatively uniform efficiencyover the entire frequency range.

A further object of the invention is to provide compact and efiicientradio frequency intercepting devices such as antennas and shields havingrelatively uniform characteristics over a wide frequency range includingfrequencies at which the dimensions of each device are commensurate withthe operating wavelength.

Another object of the invention is to provide a Faraday shield that ishighly efficient at relatively low frequencies and essentially free ofresonance effects at markedly higher frequencies.

A more particular object is to provide a Faraday shield that effectivelyblocks electric field lines of force at relatively low frequencies andremains relatively transparent to magnetic fields at frequencies ten andfifteen times higher.

It also is an object of the invention to provide a sense antenna for usein a direction finder and characterized by relatively high effectiveheight at low frequencies and relatively uniform operation up throughfrequencies many times higher than the lowest frequency of operation.

A further object of the invention is to provide a broadband array ofclosely spaced receiving antennas having substantially uniform operationand relative freedom from mutual impedance coupling and shadowing withinthe array.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing of a direction finder embodyingthe invention.

In general, the invention .provides a broadband direction finder,operating for example from below megacycles to in excess of 500megacycles. This broadband operation is made possible by the use ofantennas and shields incorporating electrically dissipative elements,principally fixed resistance elements or frequency varying resistiveelements such as a ferrite material, in series in the interceptingdevices. The resistive elements are employed in such manner that thewave intercepting devices operate 3,325,8l4l Patented June 13, 1967 withno appreciable reradiation or mutual coupling with adjacent conductorsover frequency ranges in excess of 10: 1. As a result, the directionfinder is able to detect the true source direction of interceptedsignals and thus produce correct output indications.

A direction finder sense antenna incorporating resistive elements inseries with it according to the invention has only relatively smallchanges in operating characteristics even at frequencies where itsphysical dimensions are commensurate with the wavelength. The senseantenna exhibits relatively little scattering. The resistive elements donot effectively degrade the low frequency performance of the senseantenna so that at low frequencies it has substantially the highperformance characteristic of a conventional antenna designed foroptimum operation at the low frequencies.

The invention also provides a Faraday shield that screens electric fieldfrom its interior with high efficiency at low radio frequencies and yetdoes not interfere with the operation of magnetic field sensing devicesin its interior at frequencies several times greater than the loweroperating frequencies.

More particularly, turning to the drawing, the illustrated directionfinder is constructed with a directive loop antenna 10 arranged to berotated in the vertical plane by a motor 12. The signal developed by theantenna 10 is applied through a transmission line 14, incorporating arotary joint indicated at 16, to an input port of a receiving andindiciating unit indicated generally at 18. The unit 18 is alsoconnected with the drive motor 12 to receive a signal indicating theinstantaneous rotation orientation of the loop antenna.

The loop antenna 10 employs conventional construction and, as is wellknow for such antennas, has a directinoal response to the magnetic fieldof of an electromagnetic wave such that the antenna output sign-a1 is ata null when the direction incoming signal is transverse to the plane ofthe antenna. The output signal from the loop antenna 10 thus drops toits null valve and rises to its peak valve twice during each fullrotation of the loop antenna, with the two peak signals having oppositepolarity. The loop antenna can utilize several loops of different sizesto efiiciently cover wide frequency ranges.

A stationary sense antenna indicated generally at 20 is also connectedto the receiving and indicating unit 18. The sense antenna develops asignal having the same polarity regardless of the arrival direction ofthe intercepted wave. Thus, the signals from the loop and sense antennasare in phase to reinforce each other with one orientation of the loopantenna, and, when the loop tantenna is reversed, the two signals haveopposite phase and subtract. The receiving and indicating unit 18combines the sense antenna signal and the loop antenna signal tounambiguously indicate the direction from which the loop antenna 10 hasintercepted an incoming signal.

Although the sense antenna 20 could be a single monopole disposed abovethe loop antenna 10, the present direction finder employs an array ofmonopole antennas 22 spaced in a circular path around the loop antenna10 and substantially horizontally coextensive with it. This arrangementis considerably more compact than having the sense antenna above theloop antenna and, since the monopoles are disposed on all sides of theloop antenna, most of them will be outside its electrical shadow.Dielectric supports 23 support the monopole antennas on a conductivebase plate 28.

To shield the loop antenna 10 from electric field lines of force, whichwould interfere with and detract from its direction finding operation, aFaraday shield indicated generally at 24 is disposed around the sidesand top of the loop antenna 10 and inside the array of monopoles.

The shield has a hat-like superstructure of conductive bands 26 disposedaround the top and sides of the loop antenna. Each band 26 is containedsubstantially within a vertical plane and connects to the base plate 28.

A direction finder such as shown in the drawing and having a Faradayshield that is approximately 12 inches high and approximately the samediameter and in which each monopole is approximately inches high,operates over a bandwidth extending from below 30 megacycles to morethan 500 megacycles. The monopoles are made this long to have sufficienteffective height at the low end of the frequency range. The shield islarge enough to enclose a loop 10 operating efficiently below 30megacycles. Nevertheless, the sense antenna and shield dimensions are arelatively insignificant part of a wavelength at the lower end of thefrequency range. More particularly, an eight-wavelength at 30 megacyclesis ap proximately 50 inches. At the upper end of the frequency range,however, the linear dimensions of the monopoles 22 and of the conductingbands 26 of the Faraday shield are commensurate with a fraction of thewavelength. For example, both elements are more than aquarter-wavelength in size at 300 megacycles. As a result, the monopolesand the bands 26 of the shield tend to exhibit strongresonances.However, as will now be described, they are so constructed that they donot resonate; instead, they have substantially uniform operationthroughout a broad frequency range such as the one described above.

As shown in detail for the monopale 22a, each monopole 22 is constructedwith resistive segments 30 and conductive segments '32 connected in analternate series succession. The resistive segments can be of ferritematerial to provide a frequency varying resistance or, more simply, beof conventional carbon or like resistive material. The electricallyresistive material is thus essentially distributed throughout theportion of the monopole that intercepts incoming radio waves. Theresistance of the segments 30 can thus be considered as a distributedparameter of the monopole and part of its internal im edance. Theresistive segments 30 are distributed throughout the monopole in thesense that no conductive segment 32 approaches its resonant frequency,or alternatively, is an appreciable part of a wavelength long,throughout the range of operating frequencies. Each monopole 22 can,alternatively, be fabricated with a continuous piece of resistivematerial. For ease in connecting the monopoles 22 to the receiving unit18, each group 20a, 20b, 20c, and 20d of the three adjacent monopoles isconnected in parallel by means of resistive jumpers 34. Transmissionlines 35a, 35b, 35c and 35d connect the groups of antennas to the unit18.

The total resistance of each monopole, due to the resistive segments 30therein, is intermediate the values of impedance at resonance and waybelow resonance for the same monopole having no resistive segments. Forexample, an allustrative monopole entirely of conductive material suchas copper has substantially zero ohmic resistance, a radiationresistance of approximately 40 ohms at resonance and a non-resonantimpedance of the order of 10,000 ohms. The same monopole constructedaccording to the invention has resistive segments 30 with a totalresistance of around 500 ohms. Each jumper 34 has a resistance of around50 ohms.

With this arrangement, the resistance of each monopole is sufficientlysmall to have substantially no effect on the total impedance atfrequencies Where the monopole does not tend to resonate. The addedresistance thus has a negligible effect on antenna performance at thesefrequencies. At frequencies where the monopole structure tends toreasonate, however, the monopole resistance has considerable effect,being, for example, 10 times greater than the resonant impedance of thesame size nonresistive monopole. The added resistance substantiallydegrades the performance of the monopole at frequencies in theneighborhood of resonance and thus essentially eliminates standing wavesfrom the monopole.

As a result, each monopole 22 can have sufficient height to be effectiveat low frequencies and yet at much higher frequencies it does not tendto re-radiate energy or to couple with an adjacent monopole or with theFaraday shield 24.

A further feature of the novel structure for the sense antenna 20 isthat the amplitude of the signal input to the receiving and indicatingunit 18 does not exhibit sharp fluctuations as would be the case wherethe sense antenna develops sharp resonances. Accordingly, the radiofrequency input circuit of the unit 18 can be constructed withrelatively high sensitivity and without the need for burnout protectiondevices that would diminish the sensitivity.

Turning now to the Faraday shield 24, when it is constructed withconduction bands 26 of highly conductive material such as copper orsilver, it effectively shields electric fields from its interior atrelatively low frequencies and over a considerable range. However, athigher frequencies, resonances develop in the shield and it reradiatesenergy in the same manner as a resonant antenna. Again, energy reachesthe loop antenna 10 from a number of directions and the direction of theultimate source of the energy cannot be ascertained. These unwantedresonance phenomena are substantially eliminated according to theinvention by constructing the Faraday shield with bands 26 that areelectrically resistive, as by making them with resistive strands so thateach band has a resistance, for example, of 1,500 ohms for the 30-500megacycle direction finder discussed above.

It has been found that with this arrangement the Faraday shield hasrelatively negligible magnetic shielding at loW frequencies so that theloop antenna provides substantially the same operation as when enclosedin a Faraday shield having highly conductive bands. At the higherfrequencies where the highly conductive bands tend to resonate, thelossy bands 26 in the present Faraday shield remain free of resonancesand remain essentially transparent to magnetic fields, thereby enablingthe loop antenna to operate with high efiiciency over the entirefrequency range desired for the direction finder.

As with the monopole antennas 22, the bands 26 of the shield thus havingresistive segments distributed along their lengths. The resistance ofeach band is intermediate the magnitudes of its impedance at resonanceand off resonance.

The invention thus provides lossy radio frequency Wave-interceptingdevices of sufficient size to be effective at low frequencies. Theresistance of each device is relatively small compared to the magnitudeof its reactance at the low frequencies in its operating range. Hence,the added resistance has little effect at these frequencies. At thehigher frequencies of operation where the reactive impedances of thedevices tend to decrease, the internal series resistances becomeincreasingly effective and maintain the operating characteristicssubstantially uniform. That is, at frequencies where thewave-intercepting device normally tends to be resonant, the resistancetherein dissipates the resonant currents that tend to develop, therebyenabling the device to continue operating as a nonresonant device muchthe same as it does at the low frequencies. When incorporated in thepresent direction finder, these features of the sense antenna andFaraday shield make it possible for a single direction finding structureto operate over a remarkably wide frequency range that heretoforerequired separate antenna structures that involved considerably morebulk and cost than the present direction finder.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efiiciently attained and,since certain changes may be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

Having described the invention, what is claimed as new and secured byLetters Patent is:

What is claimed is:

1. Broadband radio reception apparatus comprising in combination (A)receiving means operable over a range of frequencies extending from afirst frequency to a second frequency that is higher than said firstfrequency,

(B) antenna means (1) having electrically dissipative means distributedin series therein,

(2) being a quarter-Wavelength long at a third frequency intermediatesaid first and second frequencies,

(3) being less than an eighth-wavelength long of said first frequency,

(4) having a resistance that (a) is less than the magnitude of theantenna impedance at said first frequency, and

(b) is greater than the magnitude of the antenna impedance at said thirdfrequency.

2. Radio reception apparatus according to claim 1 in which saiddissipative means includes at least one electrically resistive segmentin series in said antenna means.

3. Radio reception apparatus according to claim 1 in which saiddissipative means is of ferrite material having a frequency dependentresistance.

4. Radio reception apparatus according to claim 1 in which said antennameans is constructed with alternate series-connected portions ofelectrically resistive material and portions of substantiallynon-resistive electrically conductive material.

5. Radio reception apparatus according to claim 1 in which said antennameans has a wave-intercepting structure of electrically-resistivematerial.

6. A radio direction finder comprising in combination (A) radioreceiving means responding to antenna signals within a range of radiofrequency extending between a first frequency and a sec-nd frequencyhigher than said first frequency,

(B) means for controlling the orientation of antenna means and providingan indication corresponding to said orientation,

(C) directive antenna means connected with said orientation controllingmeans and with said receiving means, and

(D) sense antenna means including at least one antenna element, saidsense antenna means (1) being connected with said receiving means,

(2) having electrically resistive means distributed in series in eachelement thereof,

(3) each antenna element being a quarter-wavelength long at a thirdfrequency intermediate said first and second frequencies, and being lessthan an eighth-wavelength long at said first frequency,

(4) each antenna element having a resistance that (a) is substantiallyless than the magnitude of the antenna element impedance at said firstfrequency, and

(b) is substantially greater than the magnitude of the antenna elementimpedance at said third frequency.

7. A radio direction finder according to claim 6 in which said senseantenna comprises a plurality of antenna elements disposed along aclosed path encircling and closely spaced from said directive antennameans.

8. A radio direction finder according to claim 7 (A) in which saiddirective antenna means is responsive to magnetic field lines of force,and

(B) further comprising a multiple-conductor electric field shield, saidshield being disposed around said directive antenna means and withinsaid closed path and being constructed with electrically dissipativemeans distributed in series in each conductor thereof so as to besubstantially non-resonant throughout said frequency range between saidfirst and second frequencies.

9. A radio direction finder comprising in combination (A) radioreceiving means responding to antenna signals within a range of radiofrequencies extending between a first frequency and a second frequencyhigher than said first frequency,

(B) directive antenna means connected with said receiving means andbeing responsive to magnetic field of lines of force, and

(C) an electric field shield disposed around said directive antennameans and constructed with conductive members at least some of which (1)are a quarter-wavelength long at a frequency intermediate said first andsecond frequencies and less than an eight-Wavelength long at said firstfrequency, and

(2) have electrically dissipative means distributed in series therein tohave a resistance that is greater than the magnitude of the reactance ofthe conductive member at said frequency at which it is aquarter-wavelength long.

10. A radio direction finder according to claim 9 in which said shieldis a Faraday type shield composed of a plurality of conductive bandseach of which are electrically resistive means in series therein to besubstantially nonresonant throughout said frequency range between saidfirst and second frequencies.

11. An antenna system for a radio direction finder, said antenna systemcomprising in combination (A) directive loop antenna means arranged forrotation about an axis,

(B) a Faraday shield substantially shielding said directive antennameans from electric field lines of force,

(1) said Faraday shield being composed of conductive bands each of whichhas electrically resistive means distributed in series therein.

12. An antenna system according to claim 11 further comprising aplurality of monopole sense antennas disposed along a circular patharound said Faraday shield, each monopole antenna having electricallyresistive means distributed in series therein.

References Cited UNITED STATES PATENTS 2,656,536 10/1953 Lockhart343-118 2,712,602 7/1955 Hallen 343-828 X 2,917,744 12/1959 Gray 343842X RODNEY D. BENNETT, Primary Examiner. R. E. BERGER, Assistant Examiner.

6. A RADIO DIRECTION FINDER COMPRISING IN COMBINATION (A) RADIORECEIVING MEANS RESPONDING TO ANTENNA SIGNALS WITHIN A RANGE OF RADIOFREQUENCY EXTENDING BETWEEN A FIRST FREQUENCY AND A SECOND FREQUENCYHIGHER THAN SAID FIRST FREQUENCY, (B) MEANS FOR CONTROLLING THEORIENTATION OF ANTENNA MEANS AND PROVIDING AN INDICATION CORRESPONDINGTO SAID ORIENTATION, (C) DIRECTIVE ANTENNA MEANS CONNECTED WITH SAIDORIENTATION CONTROLLING MEANS AND WITH SAID RECEIVING MEANS, AND (D)SENSE ANTENNA MEANS INCLUDING AT LEAST ONE ANTENNA ELEMENT, SAID SENSEANTENNA MEANS (1) BEING CONNECTED WITH SAID RECEIVING MEANS, (2) HAVINGELECTRICALLY RESISTIVE MEANS DISTRIBUTED IN SERIES IN EACH ELEMENTTHEREOF, (3) EACH ANTENNA ELEMENT BEING A QUARTER-WAVELENGTH LONG AT ATHIRD FREQUENCY INTERMEDIATE SAID FIRST AND SECOND FREQUENCIES, ANDBEING LESS THAN AN EIGHT-WAVELENGTH LONG AT SAID FIRST FREQUENCY, (4)EACH ANTENNA ELEMENT HAVING A RESISTANCE THAT (A) IS SUBSTANTIALLY LESSTHAN THE MAGNITUDE OF THE ANTENNA ELEMENT IMPEDANCE AT SAID FIRSTFREQUENCY, AND (B) IS SUBSTANTIALLY GREATER THAN THE MAGNITUDE OF THEANTENNA ELEMENT IMPEDANCE AT SAID THIRD FREQUENCY.